1. Field of the Invention
The present invention concerns a mode converter for a microwave power transmission circuit.
2. Description of the Prior Art
The techniques associated with high power millimetric and centrimetric waves are currently undergoing development because of generators and amplifiers such as gyrotrons, etc. The waveguides used are over-sized so as to enable the transmission of the necessary power and reduce transmission losses because the electrical fields vary in proportion to the square root of the power transmitted and in inverse proportion to the square root of the guide section.
Furthermore, if the dimensions of the guide are too great, for a given working frequency, several propagations modes can be created. This is unacceptable because of losses by conversion of unwanted modes. A compromise should be made between the dimensions of the guide and the power to be transmitted.
Present-day microwave power transmission circuits generally consist of elements each working in different propagation modes. For example, these elements may be a generator in TE.sub.02 mode, a transmission line in TE.sub.01 mode and an excited antenna in TE.sub.11 mode.
To connect these elements, therefore, the output mode of one element must be converted into the mode of the following element.
There already exists a number of mode converters. These are of different types, depending on the level of power to be transmitted.
Converters for low power levels are generally used for conversion from the rectangular TE.sub.10 mode to the circular TE.sub.01 mode. Owing to the low power to be transmitted, the guides are not over-sized. According to a first technique, the circular, output waveguide is coupled to the rectangular, input waveguide by means of holes located on the small side of the rectangular guide, so as to excite the desired mode in the circular, output guide. These devices cannot be used at high power.
Another method used to achieve conversion is to gradually change the shape of a rectangular guide to obtain a circular guide that propagates the TE.sub.01 mode.
These devices cannot be used when the circular guide is over-sized for the TE.sub.01 mode, and they are very long.
Prior art types of mode converter devices for high power are made by the cascade connection of several elements.
The shape of the rectangular section waveguide, propagating the TE.sub.01 mode, is gradually changed to obtain a monomode, circular section guide propagating the TE.sub.11 mode. The diameter of the circular waveguide is gradually increased so as to enable transmission of the necessary power. The transition from the TE.sub.11 mode to the TE.sub.01 mode is obtained by periodic changes in the shape of the walls of the guide. The periodicity of the changes in shape is equal to the beat wavelength between the input mode and the output mode of the circular guide. These converters have the drawback of being some meters long and of being costly, because it is difficult to make these changes in shape. They also have the drawback of having a very narrow pass-band.
The present invention provides a particularly simple approach for making a mode converter for microwave power transmission circuits. This device enables the conversion of the rectangular TE.sub.10 mode into the circular TE.sub.01 mode. This converter can be used to transmit high power, and the mode obtained at output has high purity. Its length is reduced, which is the goal sought.